.tri{
    box-sizing: border-box;
    width: 1px;
    border-top: 300px solid red;
    border-right: 300px solid transparent;
    border-bottom: 300px solid transparent;
    border-left: 300px solid transparent;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%,-50%);
}